Distributed optical sensing technology is proving to be suitable for a number of downhole oil and gas applications ranging from temperature sensing to passive seismic monitoring. One particularly advantageous aspect of this technology is that it enables the downhole components of the system to be passive, i.e., the electronics can be kept at the surface and not in the wellbore. As the technology evolves to develop new and improved systems for increasing performance and sensitivity, certain obstacles have been encountered. For example, fiber optic distributed sensing techniques often rely on (but do not necessarily require) monitoring of the slight backscattering of injected light from the highly-transparent fiber core. The properties of this backscattered light can provide sensitivity to various downhole parameters, such as the temperature at a specific location of the fiber. However, as the length of the fiber increases, the injected light and backscattered light suffers from increased attenuation from various loss mechanisms. This attenuation can be compensated through the use of higher power light sources, but this approach is limited by the nonlinear behavior of the optical fiber generating other unwanted effects at high laser intensities. Therefore, very weak signals can be commonplace in some distributed optical sensing systems.
When working with very weak signals, measurement noise such as noise introduced by the sensor, receiver electronics, and other unavoidable noise sources, becomes a prominent issue. For very long fiber optic lengths on ultra-deep/ultra-long oil and gas boreholes the signals could be as weak as a single photon at a time. Existing systems cannot perform adequately in this regime as they lack proper handling of noise. Furthermore, when the signal strength drops below a point where it can be described by few tens to hundreds of photons, quantum effects need to be accounted for.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.